xfstests: use a common _filter_scratch function

[Christoph Hellwig]

On Thu, Oct 21, 2010 at 02:00:08PM -0500, Alex Elder wrote:
> There are a number of tests that use a shell function called
> "filter_scratch" or "_filter_scratch" in order to have the actual
> scratch device or mount point show up in test output with a symbolic
> name.  There are two sets, each following a slightly different
> convention.  Put a common _filter_scratch function definition in
> "common.filter" and have all test scripts use that instead.
> Choosing one of the two conventions meant that a few test output
> files had to be changed.
> 
> In addition, add a call to _filter_scratch to test 185, and update
> its output accordingly.

Looks good.  It might be worth to add a TEST_DIR filter for completeness
as well.